dag: Support dynamic git clone for sub-workflows

[yottahmd]

Add support for executing sub-workflows directly from a GitHub repository by performing a full git clone. This will allow users to reference workflows stored in GitHub and execute them without manual downloads.

Example:

steps:
  - name: run sub-dag from GitHub
    run:
      repo: "https://github.com/owner/repo.git"  # Git URL
      ref: "main"                 # Optional branch, tag, or commit SHA
      path: "path/to/dag.yaml"    # Path to the DAG file in the repo
      token: "${GITHUB_TOKEN}"    # Optional, for private repositories